In recent years, the industrial utilization of porous glasses as adsorbing agents, microcarrier supports, separation films, optical members, and the like has been highly anticipated. In particular, porous glasses have a wide utilization range as optical members because of a characteristic of low refractive index.
As for a method for manufacturing a porous glass relatively easily, a method taking advantage of a phase separation phenomenon has been mentioned. A typical example of a base material for the porous glass exhibiting the phase separation phenomenon is borosilicate glass made from silicon oxide, boron oxide, an alkali metal oxide, and the like. In production, the borosilicate glass is heat-treated at a constant temperature to induce phase separation into a silicon oxide rich phase and a non-silicon oxide rich phase (hereafter referred to as a phase separation treatment), and a non-silicon oxide rich phase is eluted with an acid solution (hereafter referred to as an etching treatment), so that the porous glass is produced. The skeleton constituting the thus produced porous glass is primarily silicon oxide. The skeleton diameter, the hole diameter, and the porosity of the porous glass have influences on the reflectance and the refractive index of the light.
NPL 1 discloses a configuration in which the porosity is controlled in etching in such a way that elution of a non-silicon oxide rich phase is allowed to become insufficient partly and, thereby, the refractive index increases from the surface toward the inside. Consequently, reflection at a porous glass surface is reduced.
Meanwhile, PTL 1 discloses a method for forming a porous glass layer on a base member. Specifically, a film containing borosilicate glass (phase-separable glass) is formed on a base member by a printing method, and a porous glass layer is formed on the base member by a phase separation heat treatment and an etching treatment.
In addition, PTL 2 discloses a complex in which a resin is filled in holes in a porous glass.